1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereinafter referred to as TFT) and a manufacturing method thereof. More specifically, electro-optical devices typified by liquid crystal display devices and light emitting devices having light emitting elements and also, electrical equipment in which such devices are mounted as parts will be included in this category.
2. Description of the Related Art
In recent years, techniques of forming thin film transistors (TFTs) by using semiconductor films (of about several to several hundreds of nm in thickness) formed on a substrate having an insulating surface have been attracting attention. Thin film transistors are widely applied to electronic devices such as ICs and electro-optical devices, and especially, the development thereof as switching elements for display devices is being hastened.
Note that as display devices, liquid crystal display devices, light emitting devices and the like are known. In such display devices, driving methods such as passive matrix driving (simple matrix type) and active matrix driving (active matrix type) can be used. However, in the case where the pixel density is increased, the active matrix type in which a switch is provided for each pixel (or one dot) is considered to be advantageous in that a low voltage driving can be conducted.
In an active matrix liquid crystal display device, a display pattern is formed on a screen by driving pixel electrodes arranged in matrix. To be specific, a voltage is applied between a selected pixel electrode and an opposing electrode thereof, whereby optical modulation of a liquid crystal layer arranged between the pixel electrode and the opposing electrode is performed. Thus, the optical modulation is recognized as the display pattern by an observer.
Further, an active matrix light emitting device that uses organic electroluminescence has, in each pixel thereof, at least a TFT that functions as a switching element and a TFT that supplies a current to a light emitting element formed by sandwiching an organic compound layer between a pair of electrodes, and utilizes light emission obtained through recombination of carriers in the organic compound layer. Note that the light emitting device is expected to be applied to next-generation flat panel displays because of its features such as thinness and lightness, high-speed responsibility, and direct low-voltage drive. In particular, the light emitting device is considered to hold its superiority in a point that the device has a wider angle of view and excellent visibility compared with a conventional liquid crystal display device.
In a pixel structure in a liquid crystal display device, three of a gate wiring (scanning line), a source wiring (signal line), and a capacitor wiring each are patterned into a line shape. In this case, one of the source wiring and the gate wiring is arranged in a row direction while the other is arranged in a column direction, and plural pixels are formed in a region surrounded by both the wirings. Further, in order that both the wirings may not contact with each other at a portion where the wirings intersect with each other, the gate wiring and the source wiring are respectively comprised of conductive films formed at different layers through an insulating film. That is, there is provided such a structure in which the insulating film is formed between one of the source wiring and the gate wiring, which is formed of a first conductive film, and the other of the source wiring and the gate wiring, which is formed of a second conductive film.
On the other hand, in a pixel structure in a light emitting device, three of a gate wiring (scanning line), a source wiring (signal line), and a current supply line each are patterned into a line shape. In this case, one of the source wiring and current supply line and the gate wiring is arranged in a row direction while the other is arranged in a column direction, and plural pixels are formed in a region surrounded by the source wiring, the current supply line and the gate wiring. Further, in order that both the wirings may not contact with each other at a portion where the wirings intersect with each other, the gate wiring and the source wiring and current supply line are respectively comprised of conductive films formed at different layers through an insulating film. That is, there is provided such a structure in which the insulating film is formed between one of the source wiring and current supply line and the gate wiring, which is formed of a first conductive film, and the other of the source wiring and current supply line and the gate wiring, which is formed of a second conductive film.
However, application of such active matrix display devices (typically, liquid crystal display device and light emitting device) has been expanding, and the needs for high fineness, a high aperture ratio, and high reliability have been growing along with an increase in surface area of a screen. As a result, the needs for improvement in productivity and reduction in cost also have been growing.